1. Technical Field
The present invention relates, generally, to implementing facsimile transmissions over an IP network.
2. Background Art and Technical Problems
The transfer of facsimile documents in real-time between standard facsimile terminals over telephone lines typically conforms to the procedures and specifications in the ITU telecommunications standard generally known as T.30. These procedures and specifications were developed for the public switched telephone network, or PSTN. In that environment, if no activity occurs on the line for a certain period of time, the protocol provides that the facsimile equipment will time out. The facsimile machine will assume that the connection has been lost, and will disconnect (or hang up) when such a time out occurs. This protocol has also been used in ISDN. The T.30 protocol has very strict timing requirements.
The availability of IP networks such as the Internet providing international communications among users offers the potential of using such networks to send and receive facsimile documents. However, delays inherent in such Internet Protocol or IP networks create problems. When there is no activity on the line, the protocol will often time out. Time outs can occur over an IP network when a frame is late due to IP network delay. The facsimile machines following the T.30 protocol will time out and hang up, and consequently, the attempted facsimile transmission will fail.
Efforts to develop procedures for real-time facsimile transmission over IP networks include a draft ITU-T recommendation T.38 (06/98), the disclosure of which is incorporated herein by reference. The current draft of ITU-T recommendation T.38 (06/98) does not directly address the issue of protocol timeouts.
Attempts to deal with the problem of time outs in facsimile transmissions over IP networks have included a technique sometimes called T.30 spoofing. In this scheme, a fax over IP gateway tries to guess what T.30 frames will be sent and sends the best guesses at the right time to avoid time outs. But if the guess is wrong, the T.30 spoofing technique has to invoke a recovery scheme. The T.30 spoofing technique must follow the T.30 protocol very closely, and must take guesses at what frames to send. The T.30 spoofing technique must know how to generate frames to cancel and then re-start the T.30 negotiation, while making sure that the two facsimile machines at each end have their timing satisfied and do not hang up. The spoofing technique requires intelligence to be introduced into the gateways so that the gateways decode fax negotiation messages. Using such decoded information, the gateways must use complicated, and sometimes time consuming, schemes to try to prevent time-outs and to overcome delays in an IP network. Improved techniques are needed for preventing time-outs during fax over IP transmissions.
Thus, efforts to develop schemes to allow transmission of facsimile documents over Internet Protocol networks have not been altogether satisfactory, and significant room for improvement still exists.
In accordance with an exemplary embodiment of the present invention, a method for transmission of facsimile documents over IP networks avoids T.30 protocol time outs by sending the start of a frame preamble if a response from the remote facsimile machine has not been received in time, and by extending the preamble until the response from the remote facsimile arrives.
A method for transmission of facsimile documents over IP networks in accordance with the present invention is simpler than a T.30 spoofing scheme. All the method of the present invention needs to know is when to expect a T.30 frame so that the preamble generation can be preemptively started to avoid a time out.